Using your resources, gifts or abilities to meet someone else's needs.
Ministry usually takes the form of conversation, or waiting, or helping. It's always person to person. It's almost always ordinary and mundane. It rarely has an immediate effect, but it always has an immediate cost.
Here's a picture of what that looks like.
This is Jenn. She and her husband are in our small group. She waits on kids while they use the bathroom. She is a minister.
This is Russ (green shirt) and Karen. They stand around in the heat and make sure kids don't eat too many cookies. They are ministers.